WATER TREATMENT

WATER TREATMENT OPERATIONS The treatment of water can be classified into three categories, • Treatment of raw water for drinking purposes • Treatment of raw water specialized industrial

application • Treatment of waste water to make it

acceptable for release or reuse.

MUNICIPAL WATER TREATMENT FOR RAW WATER:

• Screening to remove relatively large floating and suspended debris.

• Mixing the matter with chemicals that encourage suspended solids to coagulate into larger particles that will more easily settle.

• Flocculation, which is the process of gently mixing the water and coagulant, allowing the formation of large floc.

• Sedimentation in which the flow is slowed enough so that gravity will cause the floc to settle.

• Sludge processing where the mixture of solids and liquids collected from the settling tank are dewatered and disposed of.

• Disinfection of the liquid effluent to ensure that the water is free from harmful pathogens.

The following factors must be taken into account for designing and operating an industrial water treatment facility. • Quantity of Water requirement • Quantity and quality of available water sources • Successive uses for applications requiring progressively lower water quality • Water recycle • Discharge standards.

TREATMENT OF RAW WATER FOR INDUSTRIAL USE:

• It is usually applied to the entire water supply • It consists of processes such as aeration,

filtration, and clarification to remove materials from water such as suspended or dissolved solids, hardness and dissolved gases from water that may cause problems.

• After this basic treatment, the water may be divided into different streams, some to be used without further treatment and the rest to be treated for specific applications.

EXTERNAL TREATMENT

Internal treatment include the following:• Addition of either hydrazine or sulfite to remove dissolved oxygen.• Prevention of formation of calcium deposits by the addition of chelatingagents to bind the dissolved calcium.• Removal of calcium by the addition of precipitants such as phosphate .• Treatment with dispersants to inhibit scale formation.• Prevention of corrosion by the addition of inhibitors.• pH adjustment.• Disinfection for food processing uses or to prevent bacterial growth in cooling water.

INTERNAL TREATMENT

Purpose : To remove the contaminants from water.

So that the treated water meet the acceptable quality standards.

The quality standards usually depend upon whether the water will be reused or discharged into a receiving stream.

Waste water treatment process consists of series of unit operations.

WASTE WATER TREATMENT:

It can be classified into,Primary treatment of waste water

Pre TreatmentPre Sedimentation

Secondary treatment for municipal waste waterBiological film systemRotating biological contactor

Oxidation ponds Activated sludge system

WASTE WATER TREATMENT PROCESS

Physical processes : screening, sedimentation, floatation and filtration

chemical processes : precipitation, coagulation and disinfection.

physical and chemical processes : air stripping, carbon adsorption, oxidation and reduction ion-exchange and membrane processes like reverse osmosis and electro dialysis are also important in certain cases.

Biological processes : bio filtration and activated sludge process.

PRIMARY TREATMENT OF WASTE WATER

It consists of screening and grit removal. Screening removes or reduces the size of trash and

large solids that get in to sewage system.After screening, the waste water is allowed to enter

a grit chamber for the removal of inorganic grit consisting of sand, gravels and pebbles.

Grit chambers are provided to protect pumps from abrasion and to reduce the formation of heavy deposits in pipes and channels.

Grit is normally allowed to settle in a tank under conditions of low flow velocity and it is then scraped mechanically from the bottom of the tank.

PRE TREATMENT

• Primary sedimentation removes both the settle able and floatable solids. and flocculent particles.

• The material that floats in the primary settling basin is known collectively as grease.

• The grease settles with the sludge and floats to the surface, which can be removed by skimming.

• This type of phenomenon is clearly observed in primary clarifiers

• The opportunity for coalescence increases with increase in bed depth, and as a result the particle removal efficiency depends on both the overflow and bed depth.

PRIMARY SEDIMENTATON

• In secondary or biological treatment, oxygen supplied to the bacteria is consumed under controlled conditions

• So that most of the BOD is removed in the treatment plant rather than in the water course.

• Thus, the principal requirements of a biological waste treatment process are an adequate amount of bacteria

• It feed on the organic material present in waste water, oxygen and some means of achieving contact between the bacteria and the organics.

• Two of the most commonly used systems for the biological waste treatment are the biological film system and the activated sludge system.

SECONDARY TREATMENT FOR MUNICIPAL WASTE WATER :

BIOLOGICAL-FILM SYSTEM

• Biological-film system also known as trickling filters• Conventional trickling filters normally consist of a

rock bed, 1 to 3 meters in depth, with enough opening between the rocks to allow air to circulate easily.

• The influent is sprinkled over a bed of packing which is coated with a biological slime.

• As the liquid trickles over the packing, oxygen and the dissolved organic matter diffuse into the film to be metabolized by the microorganisms in the slime layer. End products such as CO2, , etc.

• Diffuse back, out of the film and appear in the filter effluent.

• biological units trickling filters are affected by temperature; therefore cold weather slows down the biological activity in the filter.

• An RBC consists of a series of closely spaced, circular, plastic disks, that are typically 3.6 m in diameter and attached to a rotating horizontal shaft.

• The bottom of 40% of each disc is submerged in a tank containing the waste water to be treated.

• The biomass film that grows on the surface of the disks moves into and out of the waste water as RBC rotates.

• While the microorganisms are submerged in waste water, they absorb organics; while they were rotated out of waste water, they are supplied with needed oxygen. B

• They are easier to operate under varying load conditions than trickling filters, since it is easier to keep the solid medium wet at all times.

ROTATING BIOLOGICAL CONTACTOR:

ACTIVATED SLUDGE PROCESS :

• The most versatile and effective of all the waste treatment processes is the activated sludge process

• This sludge is heavily laden with microorganisms which are in an active state of growth.

• Air is introduced into the tank, either in the form of bubbles through diffusers or by surface aerators.

• The microorganisms utilize oxygen in the air and convert the organic matter containing N and P into stabilized, low energy compounds such as NO2- 4 SO+ 4 NH- 24 HPO ,CO2, H2O and synthesize new bacterial cells.

• The effluent from the aeration tank containing the flocculent biomass, known as the sludge, is separated in a settling tank, sometimes called a secondary settler or clarifier.

OXIDATION PONDS:

• Oxidation ponds are large, shallow typically 1-2m deep, where raw or partially treated sewage is decomposed by microorganisms.

• The conditions are similar to those that prevail in an eutrophic lake.

• The ponds can be designed to maintain aerobic conditions throughout, but more often the decomposition taking place near the surface is aerobic, while that near the bottom is anaerobic.

• Such ponds having a mixture of aerobic and anaerobic conditions, are called facultative ponds. T

• he oxygen required for aerobic decomposition is derived from surface aeration and algal photosynthesis; deeper ponds called lagoons, are mechanically aerated.

• Oxidation ponds, can be designed to provide complete treatment to raw sewage, but they require good deal of space.

• These ponds have been used extensively in small communities where land constraints are not so critical.

• They are easy to build and manage.• When less oxygen is liberated by photosynthesis which

may produce unpleasant odours.• They have the disadvantage that the effluent may not meet

the EPA secondary treatment requirement of 30 mg/L BOD5 and suspended solids.

• However they are simple and effective in destroying pathogenic organisms which make these ponds useful in developing countries.

• Oxidation ponds are also used to supplement secondary treatment and in such cases they are called polishing ponds.